In the Western World, AMD is the leading cause of natural blindness in the elderly affecting 50 million individuals worldwide and its prevalence may become greater with an increasingly elderly population. AMD manifests itself by the progressive destruction of the macula causing central vision loss. The dry form of AMD, which accounts for 90% of cases, is associated with the presence of small yellow ‘drusen’ deposits between the choroid and the retinal pigment epithelium that result in gradual vision loss. About 10-20% of patients with dry AMD go on to develop the more severe wet form. Recently, a common allelic variant of human complement Factor H (FH) has been linked to an increased risk of developing dry AMD (Day et al. (1998) Immunogenetics 27, 211-214; Klein et al. (2005) Science 308, 385-389; Haines et al. (2005) Science 308, 419-421; Edwards et al. (2005) Science 308, 421-424). This variant arises from a tyrosine/histidine polymorphism at amino acid residue 384 in the mature protein (referred to as residue 402 in some of the references above; corresponding to the T1277C polymorphism in the FH gene). About 35% of the individuals of European descent carry the disease-associated H384 allele, which increases the likelihood of developing AMD by 2.7 fold and may account for 50% of the attributable risk of AMD. In individuals who are homozygous for the risk allele, the likelihood of AMD is increased by a factor of 7.4. Recently, the H384 allele has also been associated with an increased risk of myocardial infarction and it has been suggested that atherosclerosis could contribute to macular degeneration (Kardys et al. (2006) J. Am. Coll. Cardiol. 47, 1568-1575). Interestingly, FH deficiency is also associated with type II membraneoproliferative glomerulonephritis (MPGN II), a rare renal disease, in which drusen have a similar composition to those found in AMD (Mullins et al. (2001) Eye 15, 390-395).
Factor H is a 155-kDa plasma protein that acts as a cofactor for the breakdown of complement C3b by Factor I. It is composed of 20 Complement Control Protein (CCP; also termed short consensus repeats or SCR) modules, each of approximately 60 amino acids with a compact structure. The Y384H polymorphism is located within CCP7. Factor H is believed to discriminate self from non-self by recognizing polyanionic structures on the former, such as sialic acid and the glycosaminoglycan (GAG) chains of proteoglycans (e.g., heparan sulphate (HS) and dermatan sulphate (DS)), and thus inhibit complement activation on host surfaces. Factor H has been shown to be present in retinal blood vessels in the choroid and is associated with the drusen of AMD patients. In addition, markers of complement activation (e.g. C5b-9 and C3 fragments, including iC3b) have been detected in the Bruch' s membrane and drusen of AMD patients, leading to the hypothesis that AMD results from an aberrant inflammatory process that includes inappropriate complement activation. Furthermore, it has been reported that the glycosaminoglycan heparan sulphate is present in the macula of AMD patients but not detectable in controls (Kliffen et al. (1996) Arch. Opthalmol. 114, 1009-1014).
Using a recombinant protein composed of CCPs 6-8 (including an additional N-terminal non-authentic glycine at the beginning of the Factor H derived sequence (see FIG. 7); hereinafter referred to as FHCCP6-8), we have now shown that amino acid residue 384 of Factor H is adjacent to a heparin-binding site in CCP7. Furthermore, we have shown that the AMD-associated H384 allotypic variant and the Y384 non-disease associated variant of Factor H exhibit differential binding to various targets, e.g. notably the H384 allotypic variant has been shown, for example, to bind better to a sample of the well-characterised 4th International Standard (4IS) heparin. It is thus now extrapolated that substitution of histidine for tyrosine at position 384 of mature Factor H may affect binding of Factor H in vivo to polyanionic patterns on host surfaces, potentially influencing complement activation, immune complex clearance and inflammation in the macula of AMD patients. More particularly, we postulate that the H384 allotype of Factor H may be preventing appropriate complement activation on or near to drusen by its binding to heparin-like structures thus leading to impairment in the clearance of drusen. It follows that there is a wish to identify agents which will inhibit binding of the H384 allotype of Factor H to heparin and other binding targets while not preventing the function of the Y384 allotype of that protein.